1. Field of Invention
The present invention relates to a method and apparatus for concurrently circulating the electrolyte within each cell of a battery while maintaining the electrolyte in all cells at an optimum, uniform temperature.
2. Discussion of Prior Art
Liquid electrolyte lead acid batteries are capable of much better performance than they currently provide. There are three primary deficiencies in the performance of flooded electrolyte lead acid batteries: inadequate energy density, inadequate life, and excessive maintenance requirements.
One reason for the limited performance of liquid electrolyte lead acid batteries is that the concentration of the electrolyte stratifies in the cells during charge and discharge, which degrades the battery's performance. Another reason is that the battery's chemical processes are sensitive to temperature, which is usually left uncontrolled in lead acid batteries. In order to achieve a substantial increase in the performance of flooded electrolyte batteries and significantly reduce each of the three deficiencies in the performance of lead acid batteries, both electrolyte circulation and temperature control, i.e. electrolyte conditioning, must be provided.
Electrolyte circulation
It is well-known in the battery art that a battery can deliver significantly more energy when the specific gravity of its electrolyte is homogenized by circulation. The energy output of a lead acid battery is maximized when it is discharged very slowly; the energy provided by a battery at a 20 hour discharge rate is typically more than twice the energy that would be provided at a 1.5 hour discharge rate. However, the duty cycles of electric vehicles often cause the battery to be discharged at rates as fast as 1.5 or 2 hours. The performance degradation at higher discharge rates is attributed to depleted ion concentration near the reaction zones in the battery plates. The depletion results because the mobility of the ions in the electrolyte is not sufficient to keep pace with the electrical discharge. Electrolyte circulation eliminates the ion depletion near the reaction zones, thus allowing much more energy to be delivered at higher discharge rates.
Further, the cooler, more dense electrolyte tends to concentrate at the bottom of the cell, resulting in non-uniform electrical performance and corrosion of the plates. Electrolyte circulation thus causes the plates to be used in a more even manner, significantly improving electrical performance and extending the life of the battery.
A major inconvenience in using deep-cycle, flooded-electrolyte, lead-acid batteries is the effort that is required to periodically monitor and correct the level of electrolyte in the cells. Conventional battery chargers finish the charging cycle at voltages that are above the gassing limit in order to cause the evolution of gasses that will stir the electrolyte and homogenize electrolyte concentration. The gassing depletes the electrolyte, which then must be replenished. If the electrolyte is circulated within the cells without gassing the battery, the need to replenish the electrolyte is eliminated.
A long standing problem in electric vehicles is the difficulty of determining the state of charge of the battery with reasonable accuracy and reliability. The specific gravity of the electrolyte in a cell is a reliable measure of the state of charge only after any concentration gradients in the electrolyte have had time to dissipate and the electrolyte concentration is homogenized. The specific gravity of the electrolyte can also be homogenized by providing continuous electrolyte circulation during discharge/charge cycles, which allows reliable specific gravity measurements to be made at any time.
Previous methods used to homogenize the concentration of electrolyte have included introducing gas bubbles into the cells through various devices to induce vertical circulation. U.S. Pat. Nos. 4,283,467, 3,083,253 and 4,693,947 It is difficult to control the circulation in these methods and they can cause acid vapor to be transported from the cell, which is both a hazard and results in depletion of the electrolyte.
Another approach is to remove the electrolyte from the cells and return it to different points within each cell with small peristaltic pumps. U.S. Pat. Nos. 4,221,847, 4,237,197 and 5,252,412 This method requires the transportation of the electrolyte outside of the cell through a system of pumps and tubes which is vulnerable to accidental spillage of the electrolyte.
The third approach to electrolyte homogenization is to provide a hydrostatic pump by creating a chamber in each cell with an inlet slightly above the electrolyte level, and a small outlet at the bottom of the cell. As the vehicle accelerates, the electrolyte level rises at the chamber end of the cell, spilling the more depleted and thus lighter electrolyte from the top of the cell into the chamber. The electrolyte flows downward when the vehicle is no longer accelerating, mixing with the more concentrated electrolyte at the bottom of the cell. U.S. Pat: Nos. 5,032,476 and 5,096,787 These devices function only while the vehicle is operating and the circulation produced is largely uncontrolled.
All of the previous devices have significant disadvantages which have limited their effectiveness and commercial use. The current invention induces circulation with small fluid pressure actuated pumps in each cell. This eliminates the deficiencies in the above methods: the circulation is controlled, no vapor is created, the electrolyte is not transported out of the cell, and the pumping system operates during both charging and discharging. Further, none of the previous devices concurrently provides electrolyte circulation and temperature control, and no attempt is made to eliminate the temperature differential between cells.
Electrolyte temperature homogenization
It is also well known in the battery art that the life of lead acid batteries is adversely affected as temperature rises. This phenomenon is traceable to the exponentially increasing solubility of the active materials in the acid electrolyte as temperature rises. A widely accepted relationship between life and temperature is that the life of a lead acid battery is halved for each 15 degree F. rise in operating temperature above 80 degrees F. It is not unusual to see substantial increases in cell temperatures due to losses within the battery during discharge and/or charge, especially in cells that are in the interior of a battery pack where they have little convective cooling.
While battery life decreases as temperature rises, the rate at which energy can be absorbed or discharged by each cell increases as temperature rises. Thus maximum battery life is limited by the hottest cells, while the maximum discharge rate and the charge receptivity of a battery pack is limited by the coolest cells. Therefore, battery should be maintained at an optimum temperature which maximizes both battery life and energy density.
Previous methods to control and homogenize the temperature of the electrolyte include placing cooling elements in the electrolyte above the plates with cooling medium circulated through them, which modifies the temperature in each cell and can be used to homogenize the temperature between cells. U.S. Pat. Nos. 5,432,026 and 4,007,315 There are also many examples of novel designs of the battery housing, to allow the flow of a cooling medium between and/or around the cells. These methods have various disadvantages, including the electrical hazards associated with the use of metal cooling elements and the requirement of specially designed housings. None of the methods provide for concurrent electrolyte circulation.
The cell pumps in the current invention serve not only to circulate the electrolyte but also to transfer heat between the electrolyte and the fluid that is used to actuate the pumps. This method substantially reduces the complexity and cost of cooling the cells.